1. Field of the Invention
This invention relates to a gaseous fuel fired, single-ended, fully internally recuperated radiant tube annulus system suitable for use in radiant tube heating applications, such as ferrous and nonferrous metal treatments, whereby, in addition to providing heat for transfer to the intended load, the products of combustion from the combustion of the gaseous fuel may be used to preheat the oxidant used for the combustion of the fuel and may be used for thermochemical recuperation.
2. Description of Related Art
Radiant tubes have long been used in industrial heating applications for heating a variety of materials, such as steel or other solid materials in a furnace. Conventionally, radiant tube heaters have been powered by electrical heating elements or by fuel-fired burners. Electrically heated radiant tubes basically comprise heating elements within a tube which extend into a furnace or work zone. The elements radiate heat to the tube and the tube radiates heat to the work. In high temperature heating applications, electrically heated radiant tubes are preferred because the heating elements radiate uniform heat flux to the tube and may radiate directly to the load. However, the cost of electricity often dictates that fuel-fired burners be used in place of the electrical heating elements to provide products of combustion into a tube which, in turn, will radiate heat to the work.
In conventional radiant tubes utilizing fuel-fired burners, the high-temperature combustion products are supplied into the radiant tube from one end thereof and, after having been used for heating, are then discharged from the other end thereof. It will be apparent that the discharged products of combustion still have a relatively high temperature. One radiant tube system, referred to as a single-ended radiant tube system, which has been developed to address this issue is a recuperative radiant tube burner system comprising an outer heat resistant radiant tube having a closed forward end in which is located an elongated recuperator tube which coacts with the radiant tube to define an annular exhaust passage for the flow of hot gases produced by a burner assembly disposed within the recuperator tube as taught by U.S. Pat. No. 5,241,949 to Collier and U.S. Pat. No. 4,705,022, also to Collier. U.S. Pat. No. 5,016,610 to Meguro et al. teaches a radiant-type heater having inner and outer concentric tubes and a fuel supply tube disposed within the inner concentric tube. The end of the outer concentric tube opposite the fuel supply end of the heater is closed off as a result of which combustion products resulting from combustion of the fuel from the fuel supply tube in the inner concentric tube are exhausted through the annulus formed between the inner and outer concentric tubes. U.S. Pat. No. 4,401,099 to Collier teaches a single-ended recuperative radiant tube assembly having inner and outer recuperative tube assemblies positioned in a counterflow arrangement within a radiant tube assembly whereby hot exhaust gases emitted from the burner within the single-ended radiant tube assembly are directed through a flame tube to an annular exhaust chamber located between the outer recuperative tube and radiant tube assemblies. Ambient air flowing toward the burner in an air chamber between the inner and outer recuperator tube assemblies is heated by the exhaust gases in the annular exhaust chamber. And, U.S. Pat. No. 6,321,743 B1 to Khinkis et al. teaches a method and apparatus for combustion of a fuel and oxidant in which at least a portion of a fuel and at least a portion of an oxidant are introduced into an annular region formed by an outer tubular member closed off at one end and an inner tubular member open at both ends concentrically disposed within the outer tubular member, forming a fuel/oxidant mixture. The fuel/oxidant mixture is ignited in the annular region, forming products of combustion therein. The products of combustion are then exhausted through the inner tubular member providing oxidant preheating prior to forming the fuel/oxidant mixture.
In conventional radiant tube systems employing integrated recuperators, combustion air or oxidant is preheated exclusively by heat transfer from the exhaust gases exiting the systems. As a result, temperature gradients involved in the heat transfer to the incoming combustion air or oxidant are relatively low and measures must be taken to enhance heat transfer to make the system reasonably efficient. For example, more surface area may be required to transfer the desired amount of heat. In addition, heat transfer enhancements may result in greater pressure drops through the system, particularly where such enhancements require additional features, such as fins.
In conventional radiant tube systems employing an integrated recuperator, the burner assembly typically extends back a considerable distance from the upstream end of the radiant tube, as a result of which the assembly may project a substantial distance outward from the furnace wall to which the system is attached, thereby taking up a considerable amount of space in the area around the furnace.
In conventional radiant tube systems employing an integrated recuperator, no cooling is provided to any of the radiant tube components, thereby creating the potential for overheating of the tube components and necessitating the use of expensive high temperature metal alloys and ceramics capable of withstanding the operating conditions.
At the high operating temperatures of conventional radiant tube systems, there is the potential for the formation of a significant amount of NOx emissions. Total NOx formation, in addition to high operating temperatures, is a function of residence time and, in radiant tube systems employing an integrated recuperator, the residence time at the high temperature of air, fuel and combustion products is relatively long since the combustion products must move down the entire length of the radiant tube and then return before being cooled in the recuperator. Thus, NOx is generated along the entire flow path. Traditionally, flue gas recirculation, in which exhaust gases are entrained and used to reduce the oxygen concentration of the combustion air, has been used to reduce NOx formation. The reduced oxygen concentration slows the formation of NOx and slows combustion reactions as well so that heat is released as uniformly as possible. Flue gas recirculation may be carried out internally within the radiant tube or externally where the exhaust gases pass through the recuperator before being entrained in the combustion air and reintroduced into the burner. In systems using either internal or external flue gas recirculation, NOx emissions, corrected to an O2 concentration of 3%, are in the range of about 70-100 ppm.